There are multiple parallel loops that connect the basal ganglia with various motor and cognitive circuits. High frequency stimulation of the subthalamic nucleus (STN) and dopaminergic medication dramatically improves the motor symptoms of Parkinson's disease. However, recent evidence suggests that STN stimulation can also have negative effects on speech and aspects of cognition. The long-range goal is to determine the effect of STN stimulation and medication on selected aspects of motor control in patients with Parkinson's disease six months following surgery and over the long term. The central hypothesis is that STN stimulation and medication act similarly for signs of Parkinson's disease that primarily involves motor pathways. STN stimulation and medication can have differential effects for tasks that include pathways involved in cognitive aspects of motor control. Aim 1 will determine the effects of STN stimulation and medication on tremor and rigidity during secondary tasks that increase the cognitive complexity of the task. The hypothesis is that STN stimulation will be less effective when a secondary cognitive task is used. Aim 2 will determine the effects of STN stimulation and medication on speech. The hypothesis is that STN stimulation will cause deterioration in speech intelligibility since multiple basal ganglia pathways are involved. Aim 3 determines the effects of STN stimulation and medication on internally guided limb force control. The hypothesis is that elbow force will be overestimated in the presence of STN stimulation because of a reduction in the activation of the anterior cingulate cortex. Aim 4 will determine the long-term effects of STN stimulation and medication on bradykinesia and tremor. The hypothesis is that STN stimulation and medication will still act through similar mechanisms, but that the absolute benefit will be reduced due to disease progression. These experiments will objectively quantify the efficacy of STN stimulation and medication for tremor, rigidity, bradykinesia, speech, and memory guided force control. Quantifying motor tasks that differ in their cognitive requirements will increase our understanding of both the positive and negative consequences of STN stimulation.